The doctors also claim that, within a decade, if this trend continues the number of deaths from lung cancer among non-smokers will surpass that of those who smoke.

Cancer research groups have identified particles of soot in the air as carcinogen – something that causes cancer.

According to The Times, there are more than 46,000 new cases of lung cancer every year in the United Kingdom. And only one in every 20 patients survives for more than a decade following a diagnosis.

About 90 percent of cases are linked to smoking cigarettes but doctors at the Royal Brompton Hospital and Harefield NHS Trust have seen an increase in the number of operations performed – despite smoking being at a record low.

Eric Lim, a consultant thoracic surgeon, shared that the number of patients treated at the center between 2008 and 2014 remained constant at about 310 a year. The number of those who never smoked had risen from fewer than 50 to nearly 100 a year.

The surgeon theorized that though the reasons for this change remains unclear, air pollution is likely the culprit.

But, some experts have discounted the study as being too small of a sample to be reliable and suggested that the reason for the increase is improvements in machines that can detect smaller tumors.

Stephen Spiro, a former head of respiratory medicine at University College Hospital said: "There is no good evidence that lung cancer is becoming commoner in never-smokers.

"Lung cancer will become more frequent in never-smokers as a proportion, as smoking cancers begin to decline."